Known methods for forming a gallium oxide thin film having high crystallinity on a sample on which films are to be formed include a method of forming a film using water fine particles, such as the mist CVD method (Patent Document 1). For example, this method is used as follows: a gallium compound such as gallium acetylacetonate is dissolved in an acid such as hydrochloric acid to form a raw material aqueous solution; the raw material aqueous solution is converted into raw material fine particles; the raw material fine particles are carried by a carrier gas to the film forming surface of a sample on which films are to be formed; and the raw material mist is caused to react to form a thin film on the film forming surface. Thus, a gallium oxide thin film having high crystallinity is formed on the sample.
The method described in Patent Document 1 also allows formation of a gallium oxide thin film having high crystallinity. However, when forming a gallium oxide thin film using the method of Patent Document 1, the inventors found that the thin film included unintended carbon impurities. Such carbon impurities also serve as a dopant. Accordingly, the existence of the unintended carbon impurities makes it difficult to control the doping concentration.
Further, an acetylacetonate complex has poor solubility in water. Even when an acidic solution is used, it is difficult to increase the concentration of the raw material solution. Accordingly, when forming a film at high speed, the concentration of the raw material cannot be increased. Further, water-based CVD methods, typified by the mist CVD method, are said to have lower raw material efficiency than other CVD methods and to have increases in the raw material efficiency as a challenge for commercialization.
On the other hand, Non-Patent Document 1 attempted to form a film from gallium chloride. Since gallium chloride is free of carbon and has good solubility in water, the inventors expected that use of gallium chloride would solve the above problems. Unfortunately, the inventors could form no film from gallium chloride and thus concluded that acetylacetonate and water were essential to form a film.